what dank has done wont work on a DB212 as the inside size is only 12inches so the sub only just fits as it is.
what i have done is similir to dank,
i cut an extra panal B, and
i used a router to house cut the speaker hole into one side of them.
thinning the panal to 6mm.
this let me sink the first sub into the panal, i used 4 screws on this sub in the holes that where over the sides of the inner box.
then i used the extra panal b i cut and had the routed side facing down, this meant the extra bit of the sub's rim that was sitting above the surface of the first panal, would sit into the cut out of the second one so the second panal would sit flat, then i screwed the second sub face down using short screws in the 4 holes that lined up with the other screws, and longer screws in the others to go through both subs and the wood.
the extra bit of work means i dont have to have the speakers touching at all and gives that little extra bit of clearence for the cones moving.
well, it's all together now and i have to say, i'm very very impressed.
have it in the boot of my car (528cubic litres) and it pounds very hard for the cheap subs i have in it.
gotta finish the edges of it and smooth it down then paint it.
then make another one now that i know how.